Status

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For bugs in Firefox Desktop, the Mozilla Foundation's web browser. For Firefox user interface issues in menus, developer tools, bookmarks, location bar, and preferences. Many Firefox bugs will either be filed here or in the Core product. (more info)

Close as works for me?
The anatomy of a tab has changed, making it more versatile and more easily customizable.
So you'll have to do something along the lines of the following to 'reset' it:
@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
.tabbrowser-tab, .tab-stack, .tab-middle, .tab-background, .tab-background-middle, .tabs-newtab-button {
background: transparent none !important;
border: 0 none !important;
margin: 0 !important;
padding: 0 !important;
border-radius: 0 !important;
}
.tab-background-start, .tab-background-end,
.tab-background-start::before, .tab-background-end::before,
.tab-background-start::after, .tab-background-end::after {
content: none !important;
width: 0 !important;
}
As detailed in the "Building a Theme"¹ documentation you should get the DOM Inspector² extension which allows you to see bound (is that the proper term?) elements that are not visible to the built-in inspector or Firebug.
¹ https://developer.mozilla.org/en-US/docs/Building_a_Theme
² https://addons.mozilla.org/firefox/addon/dom-inspector-6622/